76 research outputs found
(Sr/Ca)_{14}Cu_{24}O_{41} spin ladders studied by NMR under pressure
(63)Cu-NMR measurements have been performed on two-leg hole-doped spin
ladders Sr_{14-x}Ca_{x}Cu_{24}O_{41} single crystals (0-x-12) at several
pressures up to the pressure domain where the stabilization of a
superconducting ground state can be achieved. The data reveal marked decrease
of the spin gap derived from Knight shift measurements upon Ca substitution and
also under pressure and confirm the onset of low lying spin excitations around
P_{c} as previously reported. The spin gap in Sr_{2}Ca_{12}Cu_{24}O_{41} is
strongly reduced above 20 kbar. However, the data of an experiment performed at
P=36 kbar where superconductivity has been detected at 6.7K by an inductive
technique have shown that a significant amount of spin excitations remains
gapped at 80K when superconductivity sets in. The standard relaxation model
with two and three-magnon modes explains fairly well the activated relaxation
data in the intermediate temperature regime corresponding to gapped spin
excitations using the spin gap data derived from Knight shift experiments.The
data of Gaussian relaxation rates of heavily doped samples support the
limitation of the coherence lenght at low temperature by the average distance
between doped holes. We discuss the interplay between superconductivity and the
spin gap and suggest that these new results support the exciting prospect of
superconductivity induced by the interladder tunnelling of preformed pairs as
long as the pressure remains lower than the pressure corresponding to the
maximum of the superconducting critical temperature.Comment: 15 pages Latex, 13 figures. to be published in Eur.Phys.Jour.B,200
The symmetry problem in NaV2O5
We discuss the symmetry of NaV2O5 in the high temperature phase on the basis
of optical conductivity data. Conclusive information cannot be obtained by
studying the optically allowed lattice vibrations. However, intensity and
polarization of the electronic excitations give a direct indication for a
broken-parity electronic ground-state. This is responsible for the detection of
charged bi-magnons in the optical spectrum.Comment: Revtex, 2 pages, 1 postscript picture embedded in the tex
Phonon and crystal field excitations in geometrically frustrated rare earth titanates
The phonon and crystal field excitations in several rare earth titanate
pyrochlores are investigated. Magnetic measurements on single crystals of
Gd2Ti2O7, Tb2Ti2O7, Dy2Ti2O7 and Ho2Ti2O7 are used for characterization, while
Raman spectroscopy and terahertz time domain spectroscopy are employed to probe
the excitations of the materials. The lattice excitations are found to be
analogous across the compounds over the whole temperature range investigated
(295-4 K). The resulting full phononic characterization of the R2Ti2O7
pyrochlore structure is then used to identify crystal field excitations
observed in the materials. Several crystal field excitations have been observed
in Tb2Ti2O7 in Raman spectroscopy for the first time, among which all of the
previously reported excitations. The presence of additional crystal field
excitations, however, suggests the presence of two inequivalent Tb3+ sites in
the low temperature structure. Furthermore, the crystal field level at
approximately 13 cm-1 is found to be both Raman and dipole active, indicating
broken inversion symmetry in the system and thus undermining its current
symmetry interpretation. In addition, evidence is found for a significant
crystal field-phonon coupling in Tb2Ti2O7. These findings call for a careful
reassessment of the low temperature structure of Tb2Ti2O7, which may serve to
improve its theoretical understanding.Comment: 13 pages, 7 figure
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